Aluminum plates have been widely used as a support for lithographic plate. The aluminum support is generally grained, that is, the surface of the support is roughened in order to improve the adhesion between the support and the photosensitive layer and also to impart water-retaining property to a non-image area.
The graining is conducted by a mechanical process such as the sandblasting process; the ball graining process; the wire graining process; the brush graining process with both a nylon brush and an abrasive/water slurry; a process wherein an aqueous abrasive slurry is sprayed onto the surface under high pressure; or a chemical graining process wherein the surface is roughened with an etching agent comprising an alkali, an acid or a mixture thereof. Further, various processes are known such as the electrochemical graining process described in Japanese Patent Unexamined Published Application (hereinafter referred to as "J. P. KOKAI") No.54-146234 and Japanese Patent Publication for Opposition Purpose (hereinafter referred to as "J. P. KOKOKU") No.48-28123; the process combining the mechanical graining process with the electrochemical graining process, as described in, for example, J.P. KOKAI No.53-123204; and the process combining the mechanical graining process with the chemical graining process which uses a saturated aqueous solution of an aluminum salt of a mineral acid, as described in J.P. KOKAI No.56-55291.
Among these surface-roughening processes, the electrolytic surface-roughening process is desirable, since the shape of the roughened surface can be easily controlled and the roughed surface can be fine.
When the adhesion between the support and the photosensitive layer is to be improved to obtain a printing plate capable of providing a large number of prints (hereinafter referred to as "printing durability" in the electrolytic surface-roughening process, deep graining must be conducted with a quantity of electricity of at least several hundred Coulombs. However, in such a case, the non-image area is stained, because pits of 10 .mu. or larger are formed by the electrolytic surface-roughening treatment.
On the other hand, when small pits of smaller than 10 .mu. are uniformly formed on the surface with a relatively small quantity of electricity, the printing durability is reduced, even though the non-image area is not easily stained in the printing step.